1. Field of the Invention
The present invention relates to prepegs and their production and use. Prepegs are materials made of fibres and a resin, usually a liquid resin, that can be cured usually by heat. Prepegs are usually supplied to the user on a backing sheet which is removed from the prepreg to enable a stack of prepegs to be cured in a mould, an autoclave or a vacuum bag to produce a desired laminar structure.
2. Description of Related Art
The present invention relates to prepregs comprising fibre reinforcing layers encapsulated by a curable resin and provided with a backing sheet. Such prepregs may be stacked and cured to produce laminar structures that are strong and lightweight. The backing sheet provides support to the prepreg to enable handling and it may also be used as a carrier to bring the resin into contact with the fibrous material during prepreg manufacture. The laminar structures are well known and find many uses in industrial applications such as automotive, aerospace and marine applications and also in wind turbine structures such as the shells used for turbine blade production, the spars and the root ends of the spars. They are also used for sporting goods such as for skis, skate boards, surf boards, windsurfers and the like.
The fibrous material employed in the prepregs may be tows or woven or non-woven fabrics and are chosen according to the final use and desired properties of the final moulded laminate. This invention is particularly concerned with systems in which the reinforcing fibre consists of unidirectional multifilament tows such as a plurality of substantially parallel tows and each tow comprises a multitude of individual substantially parallel filaments. Examples of fibrous materials that may be used include glass fibre, carbon fibre and Aramid. The curable, typically thermosetting resin that is used will depend upon the use to which the laminate is to be put and the properties required. Examples of suitable thermosetting resins include polyurethane resins and epoxy resins. This invention is particularly concerned with prepregs employing thermosetting liquid epoxy resins.
Prepregs can be prepared by impregnating the fibrous material with the resin in liquid form. Some resins are liquid at ambient temperature and impregnation can therefore be achieved at ambient temperature. However, usually it is preferred to heat the resin to reduce its viscosity to aid impregnation. Other resins are semi-solid or solid at ambient temperature and are melted to enable impregnation of the fibrous material with a liquid resin.
The impregnation of the fibrous material may be achieved by depositing the resin on the backing layer for example, by passing the backing layer through a bath of the liquid resin and coating the resin on the backing layer by means of a doctor blade. The surface of the backing layer carrying the resin may then be brought into contact with the fibrous material and be pressed into the fibrous layer to achieve impregnation of the fibrous layer with the resin. Alternatively a moving film of resin may be brought into contact with a moving backing layer and then brought into contact with a fibrous layer in a pair of heated nip rollers. In most previous applications it is preferred to employ two layers of resin one on each side of the fibrous layer to produce a sandwich structure to which pressure is applied to cause the resin to flow into the fibrous layer to fully impregnate the layer to form a conventional prepreg, so that the layer largely contains fibres which are frilly embedded in resin and no air remains in the layer. These prepregs have a resin content in the prepreg ranging from 25 to 45% by weight of the prepreg.
Although the backing layer may be used in prepreg manufacture it is provided to support the prepreg during handling such as reeling of the prepreg and to provide protection during transport of the prepreg. It must therefore have sufficient adhesion to form a bond with the resin in the prepreg. The backing layer is however removed so that the prepregs may be formed into a stack in the mould, autoclave or vacuum bag where they are shaped and cured. It is therefore important that when the backing is removed it does not remove significant amounts of resin. It is also desirable that there is sufficient resin at the surface of the prepreg after removal of the backing sheet to get good adhesion between the prepregs in the stack that is formed in mould, autoclave or vacuum bag to avoid irregularities in the final laminar structure due to inadequate bonding between the layers.
United States Patent Application Publication 2011/0192535, the contents of which is hereby incorporated by reference, describes providing corona discharge treated backing material to pre-cut pieces of prepreg material in order to better retain the desired orientation of the fibres within the prepreg. The prepreg provided with the backing sheet is fed to a machine in which the prepreg with the backing sheet attached is cut into individual pieces of the desired size which are then formed into the required shape. The backing sheet is then removed to enable a stack of the preformed prepregs to be created in a mould.